The present invention relates to an exposure control unit including an element having electrostrictive properties and a fixed end and a deformation quantity of which is determined in accordance with a voltage applied thereto, and more particularly, to such an exposure control unit which drives a programming shutter provided with shutter blades having the function of the diaphragm by a driving source composed of the deformation produced at a free end of the electrostrictive element when the voltage is applied to the element.
In an ideal automatic exposure control for use in the programming shutter using the shutter blades having the function of the diaphragm, the shutter blades are opened concurrently with a shutter release operation so that the aperture thereof is gradually increased approximately linearly and the shutter blades are immediately closed at a time when a proper exposure is obtained.
In various programming shutters known heretofore, a sufficiently energized spring is immediately released in interlocked relationship with the depression of the shutter button so that the shutter blades are opened. In order to gradually open or increase the aperture of the shutter blades approximately linearly, a delay unit is provided in an opening mechanism for the shutter blades, or the moment of inertia of members of the opening mechanism including the shutter blades is sufficiently increased.
In other words, in the conventional various programming shutter, in order to obtain an ideal opening characteristic, a structure, an arrangement and a weight distribution of the opening mechanism or the members thereof including the shutter blades are very important factors. Accordingly, it is very difficult to design the ideal structure.
As described above, since the structure, the arrangement and the weight distribution of the opening mechanism or the members thereof are very important factors in order to determine the opening characteristic, the opening characteristic may be varied greatly due to variation of temperature or positional change of a camera.
Further, it is pointed out that the response of the shutter is deteriotated upon closure of the shutter and the exposure error is easily caused since it is required to increase the moment of inertia of the members of the opening mechanism including the shutter blades.
U.S. Pat. No. 3,518,930, which is a preceding patent relevant to the present invention, discloses that the shutter blades or the diaphragm blades are driven by the deformation produced in an elecrostrictive element or a magnetostrictive element.
The preceding U.S. patent is identical with the present invention in that when a voltage is applied to the electrostrictive element having a fixed end, the shutter blades or the diaphragm blades are driven by the deformation produced in a free end of the element. The control operation in the U.S. patent is achieved so that the application of a voltage corresponding to the brightness of a subject to be photographed to the electrostrictive element causes the aperture of the diaphragm to correspond to the brightness of the subject, or the application of a pulse voltage having an amplitude and a pulse width determined correspondingly to the brightness of the subject to the electrostrictive element causes an exposure quantity to correspond to the brightness of the subject. However, the U.S. patent does not intend or suggest that the application of a voltage increasing with the lapse of time to the electrostrictive element causes the free end of the element to be deformed gradually so that an approximately linear opening characteristic of the programming shutter is obtained in the same manner as in the present invention.